I have a box of old Agfa scientific plates no idea what they were for, and can't remember where I got them, I just opened the box in the end in daylight and they deteriorated bdly anyway. Tey are the perfect width for the one 9x12 which is missing a screen, I think approx 9 x 28 cm, so I could get 2 screens out of each plate. I'll make spare glass inserts for my Durst enlargers at the same time. As a penniless student in the earky 70's I made some from FP4 plates. There still perfect

Not sure how different in size the focus screens are compared to 9x12 plates. I'll check next time I'm in the UK.

Ian